1. Field of the Invention
The present invention relates to the improved etherification of phenols, and, more especially, to the selective etherification of phenolic compounds which can even contain a plurality of hydroxy functions.
2. Description of the Prior Art
It is known to this art to etherify phenols by means of an alkyl sulfate or an alkyl halide (Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], Volume III, page 54 (1965).
Such method can make it possible to selectively obtain a monoether, when stoichiometric amounts of reactants are used. However, the reactants used are expensive and, moreover, the reaction gives rise to the preparation of inorganic salts which present serious problems of corrosion of the apparatus and toxicity of the effluents.
Recently, published Japanese Patent Application No. 79/030,123 has disclosed that it is possible to etherify the phenol groups of dihydroxy aromatic compounds by reaction with alkyl phosphates; however, it is not possible to prevent significant dietherification reaction.
U.S. Pat. No. 3,911,022 describes a process for the etherification of phenolic compounds having one or more hydroxyl groups, by means of a saturated aliphatic alcohol having 1 to 4 carbon atoms or an ester of an alcohol of this type with a saturated aliphatic carboxylic acid, in the presence of an aliphatic tertiary amine or of a chloride, a sulfate or a carboxylate of an amine of this type. However, it has also been demonstrated that, when such process is applied to a dihydric phenol, a mixture of monoethers and diethers necessarily results.
Finally, U.S. Pat. No. 4,153,810 describes a process for the preparation of alkyl/aryl ethers by reacting an hydroxylated aromatic compound with an aliphatic alcohol, in the presence of strongly acid cation exchange resins, such process being characterized in that the reaction is carried out utilizing at least 3 mols of hydroxylated aromatic compound per mol of aliphatic alcohol. The temperature at which the process is carried out preferably ranges from 110.degree. C. to 130.degree. C. This process makes it possible to selectively obtain the monoether from a polyphenol. But a very serious disadvantage of a process of this type is in the high cost of the resins used as catalyst, and in their sensitivity to heat, which is likely to cause their degradation.